1. Field of the Invention
The invention relates to the production of composite glazings with use of films based on polyvinyl acetal and containing softening agent(s) having low tendency to creep.
2. Description of the Related Art
Composite safety glasses generally consist of two glass panes and an intermediate film connecting the glass panes. Partially acetalated polyvinyl alcohol (polyvinyl acetal), in particular polyvinyl butyral (PVB), containing softening agent(s) is used predominantly as a film material. Composite safety glasses (CSG) are used for example as windscreens or side glazings in the automotive field and as safety glazing in the construction industry.
Aliphatic diesters of triethylene glycol and tetraethylene glycol have become established as preferred softening agents for such PVB films. 3G7, 3G8 or 4G7 are used particularly frequently as softening agents, wherein the first figure denotes the number of ethylene glycol units and the last figure denotes the number of carbon atoms in the carboxylic acid part of the compound. 3G8 thus stands for triethylene glycol bis(2-ethylhexanoate), that is to say for a compound of formula C4H9CH(CH2CH3)CO(OCH2CH2)3O2CCH(CH2CH3)C4H9.
Compounds which, in a CSG intermediate film, ensure high transparency, low moisture absorption, good adhesion to the glass and sufficient low-temperature flexibility of the film are preferably used as softening agents for partially acetalated polyvinyl alcohols. Furthermore, these compounds must have sufficient compatibility with the partially acetalated polyvinyl alcohol, that is to say they must be miscible therewith in a sufficient amount without exuding again.
The compatibility of a softening agent and partially acetalated polyvinyl alcohol generally decreases with the diminishing polar nature of the softening agent. Softening agents of higher polarity are thus more compatible with polyvinyl acetal than those of lower polarity. Alternatively, the compatibility of softening agents of lower polarity increases with a rise in the degree of acetalation, that is to say with a decrease in the number of hydroxyl groups and therefore a decrease in the polarity of the polyvinyl acetal.
PVB film is produced on an industrial scale predominantly with 3G8 as a softening agent.
The moisture absorption of a PVB film and its associated resistance to the formation of defects with long-term use is basically determined by the degree of acetalation and the polyvinyl alcohol content of the PVB polymer used in the formulation.
At the same time, due to reasons of compatibility, the relatively high polyvinyl alcohol contents present in the polyvinyl acetals normally used on an industrial scale in PVB film require the use of the aforementioned, comparatively polar softening agents, which also have a comparatively low molecular weight and therefore comparatively high volatility.
A PVB film which is optimal for long-lasting architectural glazing should therefore rather be based on a hydrophobic PVB having a low polyvinyl alcohol content. Hydrophobic PVBs can be combined with less polar softening agents; that is to say they are compatible therewith.
At the same time, when using PVB film in the architectural field, high demands are placed on the mechanical behaviour of a PVB film. This concerns not only the tear strength of the film, but also the resistance thereof to slow slip (creep) from individual glass panes which are held only by the film, are sometimes a few mm thick and are therefore heavy. In particular if a CSG is only glued on the rear face in a façade support structure and the front glass panel is not provided with extra mechanical support, this can lead to defects.
The need for a PVB film which has both a low polyvinyl alcohol content (and therefore improved long-term stability) and also low tendency to creep (and is therefore suitable for lamination of thick individual glass panes) cannot be met easily however, since even films based on highly acetalated PVB tend to creep at increased temperature. Since, depending on the field of use, sustained temperatures of more than 60° C. can be observed in the CSG, both in the case of architectural and automotive applications, the use of PVB film based on PVB having a low polyvinyl alcohol content is normally ruled out.
Films having specific mechanical properties are known for example, from EP 2153989. In this case, polyvinyl acetals having a polyvinyl alcohol content of 17 to 22% by weight are used. These polyvinyl acetals do not have satisfactory creep properties.
WO 2009/047221, WO 2009/047222 and WO 2009/047223 describe the production of photovoltaic modules having films made of polyvinyl acetals and containing softening agent(s). The polyvinyl acetals used in this case likewise have a high polyvinyl alcohol content and therefore insufficient creep properties.
In US 2009/0250100, polyvinyl acetals having a melt flow index MFR of 0.8 to 2 g/10 min (at 150° C., 5 kg) are described for photovoltaic modules. This value also indicates insufficient creep properties.
Since quality guarantees of more than 20 years sometimes have to be provided in the architectural field for composite glass laminates, there is a need to provide intermediate films having reduced moisture absorption and softening agent release, yet sufficiently low tendency to creep at increased temperature.